fix(lazyload): lazy-image h is not a function

[ruochuan12]

in vue 3.x h is not function. this.$slots.default is a function.

vue 3.x refs render-function-api-change
vue 3.x docs h

• reappear
• test

// vant/packages/vant/src/lazyload/demo/index.vue
<script lang="ts">
import Lazyload from '..';

if (window.app) {
  window.app.use(Lazyload, { lazyComponent: true, lazyImage: true });
}
</script>
<template>
   <demo-block :title="t('basicUsage')">
    <lazy-image v-for="img in imageList" :key="img" :src="img">
    </lazy-image>
  </demo-block>
</teamplate>

[chenjiahan]

已发布 vant@3.6.5~